In industry, the production of hydroxylamine is usually coupled with other processes in which hydroxylamine is usually recycled for use. One example is a recycling system for hydroxylamine formation and oximation, in which an aqueous solution of a phosphate salt is used as the inorganic process solution; hydroxylamine is formed in a hydroxylamine formation zone by reducing nitrate with hydrogen gas in the presence of a catalyst in the inorganic process solution; and then the hydroxylamine is subjected to an oximation reaction with cyclohexanone to form cyclohexanone oxime. After oximation, the inorganic process solution forms nitrate ions by adding nitric acid or adsorbing NO2 (which forms nitric acid in the aqueous medium), and then the inorganic process solution is recycled to the hydroxylamine formation zone. The reactions involved in the above system are as follows:
Production of Hydroxylamine PhosphateNH4NO3+2H3PO4+3H2→NH3OH.H2PO4+NH4H2PO4+2H2OProduction of Cyclohexanone OximeNH3OH.H2PO4+C6H10O→C6H10NOH+H2O+H3PO4 Supplementation of Nitrate Ions in the Inorganic Process Solution Containing a Phosphate SaltHNO3+H2PO4−→NO3−+H3PO4 
FIG. 1 is a simplified block diagram of the conventional recycling system for hydroxylamine formation and oximation. The conventional recycling system for hydroxylamine formation and oximation, as shown in FIG. 1, comprises a hydroxylamine formation tower (10), an oximation tower (30), an extraction tower (50), a stripping tower (70) and a nitric acid absorption tower (90). In the system, a phosphate-containing inorganic process solution comprising nitrate ions, and hydrogen gas are delivered respectively via lines 101 and 103 to the hydroxylamine formation tower (10), in which hydroxylamine phosphate is synthesized. Unreacted hydrogen gas and other gases formed are discharged via a discharge line 105. The inorganic process solution containing hydroxylamine phosphate is delivered via a line 111 to the top of the oximation tower (30), and an organic solution containing cyclohexanone is delivered via lines 113 and 115 to the bottom of the oximation tower (30). The two solutions flowing in the opposite directions mix with each other and undergo oximation. The organic phase containing the produced cyclohexanone oxime is discharged from the top of the oximation tower (30) via a yield line 117, and the residual inorganic process solution is discharged from the bottom of the oximation tower (30) via a line 119. The discharged inorganic process solution is delivered via the line 119 to an extraction tower (50), in which the residual cyclohexanone oxime is removed, and then the inorganic process solution is delivered via a line 125 to a stripping tower (70), in which the residual organic impurities are removed. Finally, the stripped inorganic process solution is delivered via a line 129 to a nitric acid absorption tower (90), in which the inorganic process solution is supplemented with nitrate ions, and then the inorganic process solution is recycled to the hydroxylamine formation tower (10) for use in the hydroxylamine phosphate synthesis in the next cycle.
In the recycling system for hydroxylamine formation and oximation, after oximation of cyclohexanone with hydroxylamine is completed, the inorganic process solution discharged from the oximation tower may have residual organic impurities, which may adversely affect the reaction system. First, the organic impurities in the inorganic process solution may have a toxic effect on the catalyst for use in hydroxylamine formation, resulting in reduced activity and selectivity of the catalyst; and, as a result, the concentration and the yield of hydroxylamine cannot be effectively optimized. Moreover, when the inorganic process solution containing residual organic impurities is fed to the nitric acid absorption tower, the waste gas formed therefrom is liable to cause corrosion to the contacting materials. Therefore, finding a way to reduce the organic impurities in the inorganic process solution is a key goal in the production of high concentrations of hydroxylamine.
U.S. Pat. No. 3,997,607 discloses a method to reduce organic impurities in inorganic process solution by use of a thermal treatment. However, the waste gas formed during the thermal treatment is liable to cause corrosion to the contacting materials. Furthermore, in the case that the temperature of the thermal treatment is elevated to further reduce the total carbonyl content of the inorganic process solution, condensation between the organic matters may occur. As a result, the inorganic process solution becomes yellow, and, in addition, it becomes more difficult to remove the organic impurities from the inorganic process solution.
In another aspect, when the inorganic process solution is supplemented with nitrate ions in the nitric acid absorption tower, the residual hydroxylamine in the inorganic process solution may be degraded, causing loss of the useful hydroxylamine. Therefore, for the production of cyclohexanone oxime by using a high concentration of hydroxylamine phosphate, it is desirable to develop a recycling system for hydroxylamine formation and oximation that can effectively remove the organic impurities in the inorganic process solution and, at the same time, avoid degradation of residual hydroxylamine in the inorganic process solution.